The presence of significant quantities of H.sub.2 S or H.sub.2 S and CO.sub.2 in various "sour" industrial gaseous streams poses a persistent problem. Although various procedures have been developed to remove and recover these contaminants, most such processes are unattractive for a variety of reasons. An additional contaminant in H.sub.2 S, CO.sub.2 -containing streams is carbonyl sulfide (COS). In particular, this contaminant poses problems in conventional solvent-absorption schemes since it hydrolyzes slowly and cannot be released with the vent gas or with CO.sub.2 recovered.
While some work has been done in the area of removal and conversion of H.sub.2 S to sulfur by use of iron complexes in water gas streams, such processes generally are inapplicable directly to purification of hydrocarbon feedstock, coal gasification, or natural gas streams having high ratios of CO.sub.2 to H.sub.2 S. For example, U.S. Pat. No. 3,266,320 (Meuly, et al.) discloses the removal of H.sub.2 S from fluids passing through or leaving industrial process by adding to the polluted streams certain chelates of polyvalent metals. The chelates are added in an amount which is stoichiometrically substantially less than the amount of the pollutant, and the chelate- and pollutant-containing stream is contacted with oxygen. The preferred chelates are the iron, cobalt, and nickel chelates of acetyl acetone, cyclopentadiene, ethylene diamine tetraacetic acid, N-hydroxyethyl ethylene diamine triacetic acid, gluconic acid, tartaric acid and citric acid. The chelate is said to catalyze the reaction.
As indicated, however, this procedure is unsuited to gaseous hydrocarbon feedstock, coal gasification stream, or natural gas treatment since no provision is made for CO.sub.2 removal. More importantly, the use of oxygen with the materials contemplated would be intolerable. Finally, the amounts of chelate employed in the patent are merely catalytic, and do not give the desired degree of H.sub.2 S removal from gaseous streams.
Similarly, U.S. Pat. No. 3,622,273 (Roberts and Johnson) discloses the removal of H.sub.2 S from a gaseous stream by contacting the gaseous stream with a solution containing, by weight, from 0.005 to 20 percent of a ferric ion complex, from 25.0 to 99.945 percent of water, and from 0.05 to 10.0 percent of a buffering agent selected from the group consisting of alkali metal carbonate and alkali metal bicarbonate. According to the patentees, known complexing agents, such as nitrilotriacetic acid and ethylene diamine tetraacetic acid, present stability problems when employed in H.sub.2 S removal. To overcome these problems, the patentees specify the addition of a buffer to the treating solution, in the amount and type indicated previously. The ferrous complex resulting from reaction of the ferric complex with the H.sub.2 S may be regenerated in a separate zone and reused indefinitely.
U.S. Pat. No. 4,091,073 to Winkler describes a process in which H.sub.2 S and CO.sub.2 are simultaneously removed from a variety of gas streams. The gas stream containing the sour gases is contacted with a solution of the Fe(III) chelate of N-(2-hydroxyethyl)ethylene diamine triacetic acid in a CO.sub.2 -selective solvent. The hydrogen sulfide is converted to sulfur, the CO.sub.2 is absorbed to produce a purified gas stream, and the Fe(III) chelate is converted to the Fe(II) chelate. The process includes sulfur removal and simultaneous regeneration of the solvent and the Fe(III) chelate.